Photocatalytic oxidation of organic pollutants under visible light irradiation: from N-doped tio2 photocatalysts to the design of a continuous fixed bed reactor

2013 - 2014As a consequence of the rapid growth of population in urban areas, water use and reuse has become a major concern, leading to an urgent imperative of developing effective and affordable technologies for the treatment of water and wastewater. Traditional methods for water treatment are usually based on physical and biological processes but, unfortunately, some organic pollutants, classified as bio-recalcitrant, are not biodegradable. In this way heterogeneous photocatalysis may become an effective water treatment technology to remove organic pollutants hardly oxidised by conventional techniques. Photocatalysis represents one of the main challenges in the field of treatment and decontamination of water and air, because it is able to work at ambient temperature and atmospheric pressure. Heterogeneous photocatalysis is a catalytic process that uses the energy associated to a light source to activate a catalyst with semiconducting properties. The most common used photocatalyst is titanium dioxide (TiO2), which is able to oxidize a wide range of toxic organic compounds to harmless compounds such as CO2 and H2O. However, the following major factors limit both photocatalytic efficiency and activity of TiO2: a) the band gap of anatase TiO2 is 3.2 eV, i.e. it absorbs light in the UV region, so that only a small portion (5%) of sunlight can be used for a photocatalytic process. This is a great limitation in its use as photocatalyst for the conversion of solar into chemical energy; b) as in all semiconductors, photogenerated electron-hole couples undergo fast recombination in competition with charge transfer to adsorbed species on the catalyst surface; c) the use of slurry reactors limits the industrial applications of photocatalysis, since the necessary separation of catalyst powders after liquid phase reactions is troublesome and expensive. In this context, during this PhD project different routes have been explored to go beyond these limitations: 1. With respect to the use of visible light irradiation, doping with anions belonging to the p-block was investigated in recent years to sensitize TiO2 towards visible light, either by introducing newly created mid-gap energy states, or by narrowing the band gap itself. However, the role of titania dopants such as N, C, B, S, P, I and F is still not completely understood. The insertion of dopants in the crystalline structure of TiO2 may induce light absorption in the visible region, but also increases the rate of the undesired recombination of photogenerated charge carriers. This effect becomes relatively lower if the crystallinity of the oxide structure is higher... [edited by Author]XIII n.s

Optimized design of wastewater stream treatment processes by membrane technologies

Wastewater treatment by membrane technologies is gaining more and more importance and the relevant market is increasing. This trend is mainly justified by novel and high-performance membrane materials, a wider number of successful applications by membrane technologies and the progressive reduction of the investment and operating costs. The main drawback of membrane technology is membrane fouling, that reduces the membrane performances along the time and leads to a premature substitution of the membrane modules. In the last years, a better understanding of the fouling phenomena has sensibly increased the confidence in this technology. This is especially true for wastewater treatment processes based on membranes. In this case low operating costs are mandatory, thus the membrane modules should not be frequently replaced. This work briefly covers the theory and measurement procedures of the critical, threshold and boundary flux, with the aim of process optimization and control design. The goal is to operate membranes modules by avoiding irreversible fouling for a long period of time (several years). The importance of specific pretreatment processes, such as flocculation and photocatalysis, adopted to reduce fouling phenomena will be also discussed. Moreover, the design of advanced control systems for batch membrane and some examples of wastewater treatment (olive mill wastewater and the effluents from the tannery industry) will be reported

Enhanced visible-light-driven photodegradation of Acid Orange 7 azo dye in aqueous solution using Fe-N co-doped TiO2

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Selective Oxidation of Benzene to Phenol using Fe-N-codoped TiO2 Embedded in Monolithic Syndiotactic Polystyrene Aerogel

A polymeric composite consisting of Fe-N-codoped TiO2 (Fe-N-TiO2) dispersed into a monolithic syndiotactic polystyrene (sPS) aerogel (Fe-N-TiO2/sPS, 10/90 w/w) was used for the photocatalytic hydroxylation of benzene to phenol in presence of H2O2 to enhance the phenol selectivity and yield compared to Fe-N-TiO2 in powder form. Under UV light, Fe-N-TiO2/sPS composite aerogel showed selectivity to phenol of 43%, one order of magnitude more than the selectivity showed by Fe-N-TiO2 in powder form (4%). Under visible light irradiation Fe-N-TiO2 in powder form did not produce phenol, whereas selectivity to phenol of Fe-N-TiO2/sPS was 16%. The polymeric composite was recycled and reused up to five times without a significant decrease in photocatalytic oxidation activity in terms of benzene conversion and phenol yield, indicating the stability of the catalytic composite. Therefore, it was proved that the obtained photoreactive polymer composite could allow the development of innovative sustainable processes able to realize the selective oxidation reactions of aromatic hydrocarbons under mild conditions

Highly Robust and Selective System for Water Pollutants Removal: How to Transform a Traditional Photocatalyst into a Highly Robust and Selective System for Water Pollutants Removal

Highly porous monolithic aerogels based on ZnO photocatalyst and syndiotactic polystyrene (s-PS) were obtained by supercritical CO2 treatment of ZnO/s-PS gels. The prepared aerogels were characterized and their photocatalytic activity was evaluated using phenol and toluene as water pollutant models. The s-PS nanoporous crystalline phase, able to absorb pollutant molecules, was proven to be necessary to ensure high photocatalytic efficiency as the aerogel acts not only as a support, but also as pollutant pre-concentrator. The reusability of ZnO/s-PS aerogels is also strong showing no decrease in photocatalytic activity after six consecutive degradation trials. Finally, the aerogel matrix prevents ZnO dissolution occurring under acidic conditions and promotes a selective removal of the pollutants. The synergy between the photocatalyst and the innovative polymeric support provides the composite system with robustness, chemical stability, easy recovery after treatment, high efficiency of pollutant removal with a marked selectivity which make these materials promising for large scale applications

Electric energy saving in photocatalytic removal of crystal violet dye through the simultaneous use of long-persistent blue phosphors, nitrogen-doped TiO2 and UV-light emitting diodes

The aim of this work is to minimize the consumption of electricity associated with the light source in a photocatalytic process for the removal of pollutants from wastewater. For this purpose, an optimized visible light active nitrogen-doped TiO2 was used. To reduce electric energy consumption, nitrogen-doped TiO2 was coupled with long-afterglow blue phosphors (chemical formula: Sr2.90Eu0.03Dy0.07Al4SiO11) able to emit visible light when excited with UV light. Specifically, the experimental tests were performed using crystal violet dye as a model pollutant at 10 mg L−1 initial concentration. The experimental campaign has been carried out with the aim of assessing whether the photocatalytic activity of the nitrogen-doped TiO2 catalyst is still present in dark conditions, thanks to the light emitted by phosphors and consequently to determine the energy saving in terms of electrical consumption and related costs. After having established the optimal content of nitrogen-doped TiO2 in the physical mixture with phosphors, tests were carried out by alternating lighting periods with dark periods in order to identify the optimal irradiation time to obtain not only the total crystal violet degradation, but also the highest energy savings. In particular, the simultaneous presence of long-afterglow photoluminescent materials, nitrogen-doped TiO2 photocatalyst and UV light emitting diodes resulted in an economic and energy saving of about 40% thanks to the possibility of alternating “light” and “dark” periods

Determination of optimal operating condition in nanofiltration (NF) and reverse osmosis (RO) during the treatment of a tannery wastewater stream

The aim of this work is the search the best operating condition in batch membrane processes in respect to fouling. The examined case is the purification of tannery wastewater. In particular in this work, the treatment was performed by using two spiral wound membrane modules were used: nanofiltration (NF) and reverse osmosis (RO) membranes. A modified version of the traditional method used to measure critical fluxes of membranes, that is the pressure cycling method, was applied to measure both the critical and the threshold flux on the nanofiltration membrane. Finally, the obtained results were compared from an economical point of view with a conventional biological process to validate the membrane plant as possible alternative to conventional process. Copyright © 2013, AIDIC Servizi S.r.l